Method of annealing magnesia coated silicon-iron alloys in a vacuum



United States Patent f 3,227,587 METHOD OF ANNEALING MAGNESIA COATEDSILICON-IRGN ALLOYS IN A VACUUM Jack P. Martin, New Kensington, Pa.,assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., acorporation of Pennsyivania N0 Drawin Filed Aug. 18, 1959, Ser. No.834,420 5 Claims. (Cl. 148113) This invention relates to the productionof magnetic materials and particularly to a new and novel heat treatmentof silicon steel strip to develop preferred cube-onedge texture crystalorientation and the magnetic characteristics commonly associated withsuch strip.

In the maufacture of electrical grades of silicon steel strip for use inmagnetic cores for electrical apparatus, such as large transformers andgenerators, it is common practice to produce a strip that has improvedmagnetic properties in one direction. Such a practice enables one tosecure better magnetic properties, such as high penneability and lowcore loss, in the direction parallel to the rolling direction. Such atreatment is taught in United States Patent No. 1,965,559 to Goss.

Processing to develop maximum directional properties, in order to takeadvantage of this phenomenon, involves hot rolling of the steel to agauge of from .05 to .10 inch, followed by cold rolling and continuousstrand annealing treatments to reduce the strip to a gauge of from about.01 to .05 inch. The cold rolled continuous open annealed or normalizedmaterial does not exhibit the optimum directional magnetic properties.Primary recrystallization such as may occur during the continuous openanneal or heat treatments between the different cold rolling steps hasno substantial effect on directional properties. To impart the desiredtexture, or what is commonly known as [100] (110) or cube-on-edgecrystal orientation, with the [100] direction, i.e., a cube-edgeparallel to the rolling direction, it is necessary to heat the strip toa temperature in exces of about 1400 F. for a sufiicient time to effectsubstantial secondary recrystallization where crystals of the preferred[100] (110) texture grow in preference to other crystals. To effectmaximum directional properties, it is necessary to hold the strip in atemperature range of from about 1400 F. to 1800 F. for a sufficient timeto effect a secondary grain growth of the preferred oriented crystals.

Small, but significant amounts of impurities, such as carbon, manganeseand sulfur, are usually present in this steel since they cannot beeconomically removed or avoided in the melting process or because theyare added for metallurgical control of the alloy. Carbon is picked upfrom the pig iron and other raw materials in the melt charge. Itssubsequent removal to low levels with iron ore or oxygen during meltingprovides a source of heat and a means for accomplishing other refiningprocesses in the melting furnace. Sulfur is generally undesirable at alllevels and is controlled mainly by the use of low sulfur raw materials.The sulfur content can be further reduced during refining of the melt,but this process involves additional costs, and in some instances sulfurmay be desirable in limited amounts, since the distribution of sulfideparticles in the wrought alloy has a beneficial influence in thedevelopment of the desired texture in the grain oriented grades ofsilicon iron alloys. However, in any event, in the finalproduct, it isdesirable to have the sulfur and carbon content of all the silicon ironalloys as low as possible for the best magnetic quality. Carbon may beremoved during processing by exposing the surface of the thin sheet orstrip to a decarburizing atmosphere at a tem- 3,227,587 Patented Jan. 4,1966 perature sufiiciently high for carbon and oxygen to combinerapidly. Sulfur is not so easily removed and once the steel is cast thesulfur level cannot be substantially lowered except in very hightemperature heat treatments. A reducing atmosphere (usually dryhydrogen) and basic refractory coatings are employed for this purpose inwhich the sulfur combines with the coating and/ or with the hydrogen andis removed as hydrogen sulfide. Other elements, such as carbon, oxygen,nitrogen and manganese, may also be lowered beneficially in the hightemperature reducing atmosphere anneal.

The requirements for the complete processing of commercially preferredgrain oriented grades of silicon steel strip to obtain optimum magneticcharacteristics involve a single high temperature box anneal to atemperature range of from about 2050 F. to 2200 F. in a reducingatmosphere. It is conventional practice to box anneal coils of strip ina dry hydrogen atmosphere for periods of from 50 to hours attemperatures ranging from 1400 F. to 2200 F. In box annealing,conventional production size charges require considerable time to reachtemperatures in excess of about 1800" F., and the heating cycle is suchthat coils o f the strip are held within the 1400 F. to 1800" F.temperature range for sufficient time to effect the cube-on-edge orcrystal texture and are controlled within the range of 1800 F. to 2200F. for sufficient time to remove sulfur and the other undesirablematerials as well as cause the grain growth process. The total time ofsuch heat treatment frequently is as long as 75 hours but may be asshort as 1 hour.

Usually the strip is first given a continuous decarburizing heattreatment at about 1475" F. in a hydrogen-bearing atmosphere thatcontains sufiicient water vapor to provide a high dew point before finalbox annealing. Such treatment effects primary recrystallization but isnot of suificient duration to cause the desiredsecondary'recrystallization and does not alter the necessity for thesubsequent box anneal that is required to obtain substantially complete[100] (110) crystallization, grain growth and chemical purification. Thedecarburizing heat treatment may be employed in some other sequence thandirectly following the last cold rolling, the strip sometimes beingdecarburized prior to the first cold roll. Such a treatment may or maynot be necessary depending on the carbon content of the strip and theultimate use of the material.

It may also be desirable to short time anneal the silicon steel stripparts or magnetic cores that have been sheared or fabricated after finalorientation and heat treatment to relieve stresses imparted to thematerial by such fabrication or shearing steps. Shear stress adverselyaffects the electrical and magnetic properties of the steel. Such a heattreatment usually involves heating the material to a temperature ofabout 1450" F.

In the past, the pure dry hydrogen has been thought to be the mostsatisfactory atmosphere for high temperature annealing. This atmospherewas believed not only to serve to purify the steel by removing sulfur(theoretically as (H S), oxygen, nitrogen and carbon, but containspractically no contaminants and the small amount of hydrogen whichdissolves in this steel is not considered to be detrimental.

In performing the high temperature long time box anneal, it is necessaryto employ coatings on the surface of the steel in order to avoidadjacent surfaces of the coiled metal from fusing together at suchtemperatures. The material most commonly employed is magnesia, whereinmagnesia is applied to the surface of the silicon steel by passing thesteel through aqueous suspensions of such material (at least one of M O,hydrous M O and M OH). Magnesium oxide coatings are highly satisfactoryfor this purpose; however, they react with the silicon of the steelduring the high temperature heat treatment to form magnesiumorthosilicate glass. The magnesium orthosilicate glass is a good basefor subsequent electrical insulating coats; however, such a glass ishighly detrimental when such steel is employed for stamping largequantities of laminations, as is frequently the case. The orthosilicateglass causes excessive wear on punches and dies. Present practices ofpreparing silicon steel strip to be punched into laminations(particularly .014 inch material) is to chemically pickle theorthosilicate glass from the surface of the strip. This procedure hasproved to be highly unsatisfactory inasmuch as the resultant surface isrough and undesirable and such a pickling step is costly. Theorthosilicate glass clings tenaciously to the steel and is much moredifiicult to remove than ordinary iron oxide scale.

It has now been found that the final high temperature box anneal may beefiected, While reducing the sulfur content and other impurities to thedesired level, without the formation of the magnesium orthosilicateglass that is so highly detrimental for subsequent stamping operations.

It is therefore the object of the present invention to provide a methodof box annealing electrical grades of silicon steel strip within thetemperature range of from 1400 F. to 2200 F. without the formation of amagnesium orthosilicate glass coating,

It is also an object of the present invention to provide a new method ofannealing electrical grades of silicon steel strip, other than a hightemperature reducing atmosphere box anneal.

Further objects and advantageous features of the present invention willbe obvious from the following de tailed description:

In general, the present invention relates to the substitution of the dryhydrogen atmosphere of the conventional final box anneal with theprocess of effecting the final box anneal in a substantial vacuum.

It has been thought in the past that the use of a reducing atmosphereand particularly a hydrogen atmosphere was necessary for sulfur removalduring the high temperature anneal of electrical grades of silicon steelstrip inasmuch as it has been assumed that sulfur migrating from thesteel during the anneal is necessarily removed from the surface of thesteel as H 8 and that, therefore, hydrogen must be present to permitthis reaction. However, it has now been determined that the final boxanneal may be elfected in a substantial vacuum while substantial andsignficant amounts of sulfur as Well as other undesirable impurities andresidual elements are removed in a corollary manner.

It is a well known fact that the high temperature heat treatment causesexcessive gain growth which though desirable for magnetic properties isundesirable from the standpoint of mechnical properties, particularlywhere subsequent forming steps of the metal are to be taken. Therefore,from the standpoint of desired mechanical properties, it would be highlydesirable and may be the practice to effect the high temperature boxanneal at some time prior to the final cold rolling and annealing cycle.

Although optimum magnetic properties may not be obtained in strip thathas been subjected to the grain orientation and purification treatmentprior to final cold rolling, such procedure may be followed to secureoptimum mechanical properties. Such treatment would obviously have to beeffected after final hot rolling because hot rolling after securingpreferred orientation would alter the desired cube-on-edge structure. Inany event, the advantages of the present invention would be as equallydesirable and effective as when applied to such an anneal when it isperformed as the concluding treatment of strip preparation. Coating toprevent fusion of adjacent metal surfaces during the high temperaturebox anneal is necessary regardless of the sequence of the processingsteps. The removal of a glass presents an equal or greater problem inthis event, because such a coating would have to be completely removedbefore subsequent cold rolling would be practical. By providing asubstantial vacuum instead of hydrogen when performing the hightemperature treatment prior to final cold rolling, the steel surface iseconomically and easily prepared for the subsequent fabrication.

Very thin oriented steels of 1-6 mils thickness are produced by thefurther cold rolling of full finished hydrogen annealed oriented steelin a range of thickness of .0075 to .025". Again the hard abrasivesurface produced in the hydrogen anneal results in a costly and usuallyunsatisfactory stripping operation. The substitution of a vacuum annealin place of the hydrogen anneal of the starting material offers apractical cost saving solution to the problem of surface preparation ofthese products.

The coating applied to the surface of the steel, to separate it andprevent welding of adjacent steel surfaces at high temperatures,contains considerable amounts of combined water since it is best appliedas a Water slurry. The presence of moisture in the annealing atmospherecan effectively retard the sulfur removal process when this atmosphereis hydrogen. Annealing under vacuum will remove all moisture at a muchfaster rate so that it will not be present during the high temperatureportion of the annealing cycle. To accomplish these conditions in ahydrogen anneal requires that the anneal be held at a low temperaturefor several hours to permit the moisture to be swept out by the hydrogenatmosphere after the coating has reached a temperature at which thechemically combined water is released to the atmosphere. Under vacuumthis temperature will be much lower. The moisture would be removed at afaster rate since it is positively evacuated instead of being merelydiluted with dry hydrogen.

The vacuum employed as a substitute for the dry hydrogen atmosphere inthe high temperature box anneal of electrical grades of silicon steelstrip may vary considerably during annealing since gas will be given offby the steel; however, such a vacuum must be maintained at a level thatwill avoid oxidation and that will, in fact, promote gaseous emissionfrom the metal. Therefore, it is necessary that the vacuum be asubstantial vacuum but, of course, may vary. For example, it would notbe practical to carry out the process of the present invention where thepressure in the annealing furnace exceeded 1000 microns of mercury. Itis preferred, however, to employ vacuums of 100 microns of mercury andless.

The following specific examples are given to illustrate the effects ofthe employment of the present invention and in no way limit theinvention to the exact embodiments set forth in Table I. Table I showsthe comparative results of annealing coiled samples of magnesia coatingelectrical grades of silicon steel strip in hydrogen atmosphere and insubstantial vacuums ranging in pressures of from 60 to about 200 micronsof mercury, the temperatures ranging from about 70 F. to about 2150 F.and were within the range of 1400 F. to 2150 F. for approximately 40hours. At the conclusion of the testing, the separating medium (MgO) ofthe material subjected to the vacuum anneal was granular and loose withno apparent adhesion to the steel surface. It was readily removed indilute sulfuric acid (10%). The steel subjected to the hydrogenatmosphere anneal exhibited a tenacious glass coating at the conclusionof the anneal which required repeated cycling through acid picklingbaths.

Table I 60 cps. Magnetic Properties, and percent C, S, and

Mn after a Hydrogen Anneal at 2,150 F. of Grain Thick Oriented SiliconSteel Lot No. ness Inches Watts/lb. B at n at Pcr- Pcr- Per- 15000B 10H200B cent cent S cent Mn Inches 532473 014 586 17, 940 8400 0067 004 03953-2480- 014 619 17, 850 8260 0034 004 040 52-7248"- 014 827 16, 0505800 0035 005 057 52-7206 012 552 18, 170 8770 0055 004 0-13 Z7194 012649 16, 880 7550 0033 003 051 532573 012 717 16, 280 6150 0029 005 05160 cps. Magnetic Properties and percent C, S, and Mn after a VacuumAnneal at 2,150 F. of Grain Thick- Oriented Silicon Steel Lot No. ness,

Inches,

Watts/1b. B at a at Per- Per- Per- 1500013 H 200B cent cent cent 0 S MnCHELHCAL ANALYSES OF SAMPLES AS CAST BEFORE ANNEALING 1. In the processof making magnetic strip material wherein silicon steel strip is coatedwith at least one material selected from the group consisting of MgO,anhydrous MgO and MgOH, coiled and box annealed within the temperaturerange of from 1800" -F. to 2200 F., the improvement comprising,conducting said box anneal in an atmospheric pressure of 1000 microns oimercury or less.

2. In the process of making magnetic strip material wherein siliconsteel strip is coated with at least one material selected from the groupconsisting of MgO, anhydrous MgO and MgOH, coiled and box annealedwithin the temperature range of from 1800 F. to 2200 F, the improvementcomprising, conducting said box anneal in an atmospheric pressure ofmicrons of mercury or less.

3. In the process of making magnetic strip material wherein siliconsteel strip is coated with at least one material selected from the groupconsisting of MgO, anhydrous MgO and MgOH, coiled and box annealed at atemperature of at least 1800 F., the improvement comprising, conductingsaid box anneal in an atmospheric pressure of 1000 microns of mercury orless.

4. In the process of making magnetic strip material wherein siliconsteel strip is coated With at least one material selected from the groupconsisting of MgO, anhydrous MgO and MgOH, coiled and box annealed at atemperature of at least 1800 F, the improvement comprising, conductingsaid box anneal an atmospheric pressure of 100 microns of mercury orless.

5. In the process of making magnetic strip material wherein siliconsteel strip is coated With a magnesium oxide slurry, and box annealed ata temperature of at least 1800 F., the improvement comprising conductingsaid box anneal at a pressure not exceeding 0.20 micron of mercury.

References Cited by the Examiner UNITED STATES PATENTS 1,110,010 9/1914Ruder 1481l3 1,973,525 9/1934 Dahl et a1. 148-113 2,389,497 11/ 1945 Gat1481 13 2,535,420 12/1950 Jackson 148111 2,992,951 7/ 1961 Aspden148-111 2,992,952 7/1961 Assmus et a1. 148-111 FOREIGN PATENTS 1,009,2145/1954 Germany.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, MARCUS U. LYONS,

Examiners.

1. IN THE PROCESS OF MAKING MAGNETIC STRIP MATERIAL WHEREIN SILICONSTEEL STRIP IS COATED WITH AT LEAST ONE MATERIAL SELECTED FROM THE GROUPCONSISTING OF MGO, ANHYDROUS MGO AND MGOH, COILED AND BOX ANNEALEDWITHIN THE TEMPERATURE RANGE OF FROM 1800*F. TO 2200* F., THEIMPROVEMENT COMPRISING, CONDUCTING SAID BOX ANNEAL IN AN ATMOSPHERICPRESSURE OF 1000 MICRONS OF MERCURY OR LESS.